Mechanisms of mechano-chemical rupture of blood clots and thrombi Prashant K. Purohit, John L. Bassani, Valeri Barsegov and John W. Weisel The goal of this proposal is to explore and understand the fracture toughness of blood clots and thrombi, thus providing a mechanistic basis for life-threatening thrombotic embolization. A combination of experiments, theoretical modeling and computer simulations will reveal how mechanical stresses (due to blood flow) in synergy with enzymatic lysis induce structural damage from the molecular to continuum scales and affect the propensity of a clot to embolize. The specific aims of this proposal are: (1) Measure and model fracture toughness of fibrin gels in quasi-static conditions, (2) Investigate rate dependent dissipative effects on toughness of fibrin gels, and (3) Study the effects of blood cells, prothrombotic blood composition, and fibrinolysis on rupture of blood clots. In Specific Aim (SA) 1, we will measure toughness of fibrin clots and provide a structural basis for rupture at the micron and nanometer scales. In SA2, we will delve into the thermodynamics and rate-dependence of the fracture of fibrin gels, including fluid flow through pores and fluid drag on fibrin fibers to capture how energy dissipation increases toughness. In the translational SA3, we will investigate toughness of physiologically relevant clots with effects of platelets, red blood cells, and neutrophils in the absence and presence of the physiological fibrinolytic activator (tPA). We will also study the rupture of clots made from the blood of venous thromboembolism patients to explore the effects of (pro)thrombotic alterations of blood composition on clot mechanical stability. Our preliminary studies show that i) the toughness of cross-linked fibrin gels is in the range of those for synthetic hydrogels, ii) the addition of tPA to a crack tip reduces the loads for crack growth, iii) fibers are aligned and broken along the tensile direction at the crack tip, and iv) crack propagation results from the rupture of covalent and non-covalent bonds. We also developed v) dynamic force spectroscopy in silico to mechanically test fibrin fibers and fibrin networks using pulling simulations and vi) atomic stress approach to map the stress-strain fields using the output from simulations. We will use continuum and finite element models of swellable biopolymer hydrogels, and statistical mechanical models for the forced unfolding of fibrin molecules. We will employ multiscale computational modeling based on Molecular Dynamics simulations of atomic structures of fibrin fibers, and Langevin simulations of fibrin networks accelerated on Graphics Processing Units. The proposed experiments cover the whole gamut of macroscopic tensile tests, shear rheometry, electron microscopy and confocal microscopy to visualize and quantitate the structural alterations of ruptured blood clots. Our experiments and modeling will help us to understand the mechanisms of thrombotic embolization and will address the clinically important question: why is there a strong association between clot structure/mechanical properties and cardiovascular diseases? The new knowledge will also help to design new hydrogel-based biomaterials that are currently at the forefront of research in mechanics, materials science and bioengineering.